The present invention relates generally to the beneficiation of clay and, more particularly, to an improved method for bleaching kaolin clay in such a manner as to facilitate the subsequent dewatering, at least partially, of the kaolin clay slurry in the dispersed state via membrane filtration thereby eliminating the clay loss commonly encountered during dewatering via conventional mechanical techniques.
Kaolin clay has many known applications in industry including use as a filler in papermaking, a coating for paper, and a pigment in paint. However, crude kaolin clay typically contains various impurities which cause discoloration. Additionally, crude kaolin clay is typically too abrasive for direct use in products. Therefore, it is necessary to beneficiate the crude kaolin clay to increase the brightness of the kaolin by removing discoloration impurities and to decrease its abrasiveness by reducing the particle size of the kaolin particles.
In general, such processes for beneficiating crude kaolin clay require that the clay be refined via wet processing as a low solids slurry. Therefore, it is necessary to add substantial amounts of water to the dry crude kaolin clay to, form a clay suspension or slurry having a low solids content, generally below 50% solids by weight and typically in the range of 15% to 40% solids by weight. However, for commercial applications, the beneficiated clay slurry must have a much higher solids content. Typically, beneficiated kaolin clays are shipped commercially for use in paper making, paper coating and paint making as a high solids slurry having a solids content in the range of 65% to 75% by weight. Therefore, most of the water added to the dry kaolin clay must be removed in order to concentrate the clay solids.
In conventional prior art methods for refining kaolin clay via wet-processing, the crude kaolin clay is dispersed in water, usually with the aid of a dispersing agent, to form a flowable aqueous suspension or slurry. Typically, the aqueous crude clay suspension is then subjected to a fractionation operation which is conventionally carried out by centrifugation of the dispersed aqueous clay slurry. Typically, the aqueous kaolin slurry is fed to the centrifuge at a solids content in the range of 45% to 50% solids. The fine particle-size fraction, generally 90% finer than 2 micrometers equivalent spherical diameter (E.S.D.) in particle size, is recovered as a more dilute dispersed aqueous clay slurry, typically having a solids content ranging from 30% to 40% solids by weight, while the coarser fraction is discarded.
Following fractionation, the recovered aqueous clay slurry may be passed through a magnetic collector to remove at least a portion of any iron-based impurities therefrom. Such impurities discolor the kaolin and, if not removed, reduce the brightness of the beneficiated end product. Alternatively, but usually in conjunction with and subsequent to the magnetic separation operation, the aqueous clay slurry is subjected to a bleaching step to remove insoluble iron impurities by reducing the iron therein from the insoluble ferric state to the soluble ferrous state. In conventional prior art practice, the aqueous kaolin clay slurry is chemically flocculated to increase bleaching effectiveness, typically by acidifying the aqueous kaolin clay slurry prior to the bleaching operation by admixing therewith an aqueous acidic solution, such as dilute sulfuric acid, in an amount sufficient to reduce the pH of the aqueous kaolin slurry to a level in the range of 2.5 to 3.5. Additionally, the solids content of the aqueous kaolin clay slurry is typically reduced to a level of 20% to 30% solids by weight prior to the bleaching operation. The bleaching is carried out by contacting the aqueous kaolin clay slurry with a bleaching agent. The bleached kaolin clay slurry is now fully beneficiated and must be dewatered, rinsed and further dried to bring the kaolin slurry to commercially acceptable levels.
To dewater the beneficiated kaolin clay slurry by conventional practice, the low solids slurry is typically first passed to a mechanical filter or an electrofilter wherein a limited portion of the water is removed from the slurry. Conventional filters customarily used to carry out this initial dewatering include hydrocyclones, filter presses, various electrofilters utilizing electrokinetic phenomena such as electrophoresis and electroosmosis, and, most commonly, rotary vacuum filters. Rinsing of the filter cake is also conventionally carried out in conjunction with the dewatering process.
Typically, the rinsed kaolin clay slurry from such a conventional filter has a solids content ranging from 50% to 60% solids and must be further dewatered by thermal techniques to yield a shippable product. Prior to such further dewatering, the acid flocculated slurry must be redispersed. To do so, the pH of the slurry is adjusted to a pH of 6 to 7.5 by the addition of one or more of a number of conventional, commercially available dispersing agents, also known as deflocculants, such as sodium hydroxide, sodium silicate, sodium carbonate, sodium metaphosphate, and sodium polyacrylate. Unfortunately, residual by-product salts and residual dispersing agent remaining in the slurry being subjected to thermal drying are known to cause an undesirable degradation in the brightness of the thermally dried kaolin particles.
In thermal drying via conventional spray drying practice, at least a portion of the partially dewatered kaolin clay slurry is passed through a spray dryer or other contact-type evaporator such as a gas-fired kiln, wherein the kaolin clay slurry is contacted with a drying gas having a temperature of 1000.degree. F. or more, such as hot air or hot flue gas generated from the combustion of a fossil fuel, typically natural gas. It is customary to pass only a portion, typically about 30% to 50%, of the kaolin clay slurry through the spray dryer and then re-mix the thoroughly dried clay slurry from the spray dryer with the remaining portion of the partially dewatered kaolin clay slurry in a high shear mixer to produce a product kaolin clay slurry having a solids content of 65% to 75%.
Alternatively, as disclosed is commonly assigned U.S. Pat. No. 4,687,546, the partially dewatered beneficiated kaolin clay slurry from the preliminary filtering step may be further dewatered by evaporating water therefrom by passing the aqueous kaolin clay slurry through one or more non-contact evaporative heat exchangers in heat exchange relationship with a heating vapor comprising water vapor previously evaporated from the clay slurry. In this manner, an energy efficient process is provided for concentrating a beneficiated aqueous clay slurry in that use is made of the heat normally wasted when the flue gas from the clay during the spray drying process, is vented to the atmosphere. Further, by using indirect heat exchange between the aqueous clay slurry and the heating vapor as a means of evaporating water vapor from the clay slurry, the formation of agglomerates typically encountered in direct contact with the heating gas is avoided and no degradation in brightness is experienced.
An alternative method of partially dewatering the kaolin clay slurry prior to thermal drying is disclosed in commonly assigned co-pending application Ser. No. 409,839, filed Sep. 20, 1989 now abandoned. As disclosed therein, an aqueous slurry of beneficiated kaolin clay is partially dewatered via membrane filtration prior to thermal drying. The aqueous clay slurry is passed along a semi-permeable membrane, while imposing a pressure differential across the membrane surface whereby water molecules are caused to pass through the semi-permeable membrane thereby increasing the concentration of the solids in the kaolin slurry in the direction of flow as the kaolin passes along the membrane. The semi-permeable membrane employed in the membrane filter is substantially impervious to the passage of clay solids therethrough, but pervious to the passage of water molecules, ions and dissolved salts therethrough. The aqueous clay slurry is discharged from the membrane filter separately from the filterate liquid as a more concentrated aqueous slurry of beneficiated kaolin clay particles. The filtrate liquid so removed from the aqueous clay slurry is substantially free of carry over clay solids. Residual salts from the bleaching process pass through the membrane filter with the filtrate liquid and therefore are not present in the kaolin clay when it is thermally dried. The presence of such residual salts in kaolin clays that have been partially dewatered on a rotary vacuum filter results in an undesirable degradation in the brightness of the beneficiated kaolin clay during thermal drying wherein the clay particles are directly contacted with a hot drying gas, such as during spray drying.
It is an object of the present invention to provide a method for wet processing a crude kaolin clay to produce a high brightness kaolin clay product wherein the kaolin clay is beneficiated by bleaching in the acid flocculated state followed by partial dewatering in a dispersed state prior to thermal drying.